Electrical bushing assembly

ABSTRACT

An electrical bushing assembly including an insulating body member having an axial opening or bore, and first and second serially connected fuse members, each having a fuse tube with electrodes at the ends thereof, with the first and second fuse members being disposed in the axial bore of the insulating body member. A flexible insulating tubular member is disposed in a tight-fitting manner to cover the exposed electrodes at the intersection of the serially connected fuse members, and an insulating coating is disposed on the first fuse member, between the first fuse member and flexible insulating tubular member, such that the coating bridges the intersection of the fuse tube and electrode covered by the flexible insulating tubular member.

United States Patent inventor Donald J. Ristuccia Athens, Ga.

App]. No. 75,181

Filed Sept. 24, 1970 Patented Jan. 4, 1972 Assignee Westinghouse Electric Corporation Pittsburgh, Pa.

ELECTRICAL BUSHING ASSEMBLY 4 Claims, 2 Drawing Figs.

US. Cl 337/224, 337/202, 337/229 Int. Cl 01h 85/00 Field ofSearch 337/161, 202, 205, 209, 222, 224, 229, 235, 292

References Cited UNITED STATES PATENTS 2,337,353 12/1943 Smith etal 337/224 X 2 9 2 7 few??? 211?". l

3,244,838 4/1966 Astleford 3,309,477 3/1967 Bronikowski ABSTRACT: An electrical bushing assembly including an insulating body member having an axial opening or bore, and first and second serially connected fuse members, each having a fuse tube with electrodes at the ends thereof, with the first and second fuse members being disposed in the axial bore of the insulating body member. A flexible insulating tubular member is disposed in a tight-fitting manner to cover the cxposed electrodes at the intersection of the serially connected fuse members, and an insulating coating is disposed on the first fuse member, between the first fuse member and flexible insulating tubular member, such that the coating bridges the intersection of the fuse tube and electrode covered by the flexible insulating tubular member.

lllllllll PATENIED JAN 41912 FIG. 2

ELECTRICAL BUSHING ASSEMBLY BACKGROUND OF THE INVENTION I 1. Field ,ofthe Invention Theinvention relates in general to electrical insulating bushing assemblies for electrical apparatus, such as transformersQand more specifically tolelectrical insulating bushings of the type which have electrical fuses mounted therein.

2; Description of the Prior Art Electrical distributiontransformers often are protected from overloads and short circuits in the connected load, by a circuit breaker disposed in the transformer tank which is connected in the secondary winding of the transformer. The electrical distribution system isprotected against faults in the transformer by a protective link or fuse connected in the highvoltage-orprimary winding of the transformer. In some instances, the circuit breaker is not used, and the protective link is selectedto protect the transformer against external overloads and short circuits,,and also to protect the electrical distribution system against faults within the transformer. In either arrangement, the use of a currentdimiting fuse in series with theprotective link is desirable, as the fault currents available in modern electrical distribution systems may exceedthe interrupting capacity of the protective link. If the interrupting capacity of the protective link'is exceeded, in the absence of a current-limiting fuse, the circuit may not be cleared by the protective link, and/or the transformer cover may be blown off due to the internal pressures generated by the fusible link.

U.S. Pat. No. 3,244,838, assigned to the same assignee as the present application, discloses coordinated current-limiting and protective link fuses serially connected and mounted in the bore of a high-voltage bushing, which structure has been successfully used for many years at a distribution voltage level of 14.4 kv.

The electrical utilities have recently been converting to higher distribution voltages, such as 22 kv., and it would be desirable to use the same basic structure disclosed in the hereinbefore mentioned U.S. Pat. for the high-voltage bushings of the transformers constructed for this higher voltage level, as a major redesign of the high-voltage bushing would substantially increase its cost, as well as complicate the stocking of the component parts. However, when the bushing structure disclosed, in the hereinbefore mentioned U.S. Pat. was extended to 22 kv., using the appropriate current-limiting fuse and protective link for this voltage, and the assembly was tested at a 25 kv. test voltage, corona discharges were created which resulted in an objectionably high level of radio interference. The usual remedies for corona discharges would suggest increasing the radial and axial clearances in the bushing, changing to nonstandard electrodes in the current-limiting fuses, or filling the axial bore of the bushing assembly with mineral oil, but all of these approaches are undesirable as they increase the manufacturing cost of the bushing.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved electrical insulating bushing assembly of the type which utilizes serially connected current-limiting and protective link-type fuses mounted in the axial bore of a bushing assembly, which enables a bushing assembly heretofore used at 14.4 kv. to be successfully operated at 22 kv. without exceeding the maximum allowable radio interference voltage level.

When a current-limiting fuse and protective link are serially connected, it is necessary to provide a flexible insulating tubular member about the electrodes of the fuses at their juncture, which fits the adjoining ends of the two fuses in a tight'fitting manner. This flexible, tight-fitting insulating tubular member prevents the ionized gas bubble generated by the operation of the protective link, from engulfing the still live lower electrode or ferrule'gof the current-limiting fuse, and ground. It was found that while the flexible tubing is substantially tight fitting, having been shrunk into place by the application of heat to the tubing, that air is trapped in voids between the flexible tubing and current-limiting fuse. Electrical stress is transferred into this air space, away from the materials which have a higher dielectric strength than air, such as the material of-ivhich'the flexible insulating tubularmember is formed,- due to the higher dielectric constant of the flexible'tiibular member than the dielectric constant of air. Thus, the'el'ectrical breakdown strength of the trapped air is exceeded when the bushingis operated at 22 kv. and above, resulting in excessive radio interference voltages.

The present invention solves this problem by acoating of insulating material which is disposed on the outer surface of=the current-limiting fuse, adjacent to and bridging the junction of its fuse tube and lower ferrule. This coating of insulating material smooths the contour of the fuse at this point, enabling the flexible insulating tubular member to more closely conform to the surface of the current limiting fuse, and the dielectric constant of the insulating coating isselected to be at least as large as that of the flexible t'ubing,transferring electrical stress away from the corner of the ferrule to a location outside the flexible insulating tube where the electrical gradient is below the breakdown strength of air.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood,- and furtheradvantages and uses thereof more readily apparent, when considered in view of the following detailed description of exemplary embodiments, taken with the accompanying drawings, in which:

FIG. 1 is an elevational view, in section, of an electrical transformer having a bushing assembly of a type which may utilize the teachings of the invention; and

FIG. 2 is an elevational view, partially in section, of an electrical bushing assembly constructed according to the teachings of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in' particular, there is shown a transformer 10 of the distribution type which has a high-voltage bushing assembly 12, which may be constructed according to the teachings of the invention. Transformer 10 includes a magnetic core-winding assembly '14, shown schematically, disposed in a metallic casing or tank 16, which is filled to a level 18 with an insulating and cooling liquid dielectric, such as mineral oil. The level 18 is sufficient to completely immerse the magnetic core-winding assembly 14 in the liquid. Magnetic core-winding assembly 14 includes a primary or high-voltage winding 24, and a secondary or lowvoltage winding 26, disposed in inductive relation with the magnetic core 28.

High-voltage bushing assembly 12, which is sealingly disposed or mounted through an opening in the casing 16, such as through an opening in the cover 20 of the casing 16, includes a terminal 22 adapted for connection to a source of external electrical potential, i.e., the electrical distribution system, and terminal 22 is connected to high-voltage winding 24 via first and second serially connected fuse members 30 and 32, respectively. Fuse members 30 and 32 are coordinated to protect the external electrical distributionsystems from faults in the transformer, with fuse 32 being a protective link of the oil-immersed type, which is selected to interrupt the electrical circuit on overload, up to a predetermined current magnitude, and fuse 30 is a current-limiting'fuse which interrupts the current at magnitudes above said predetermined current magnitude. The low-voltage winding 26 is connected to low-voltage bushings disposed on the sidewall or cover portion of the casing 16, either directly, or via a circuit breaker, as desired.

Electrical bushing assembly 12 is illustrated in greater detail in FIG. 2, which is an elevational view, partially in section, of bushing assembly 12, illustrating the teachings of the invention. More specifically, electrical bushing assembly 12 includes a substantially cylindrical, elongated insulating body member 40, which may be formed of porcelain, or other suitable insulating materials, such as molded or cast insulating systems of the epoxy or polyester type, with the body member 40 having first and s'econd'ends'42and 44,.respectively, an axially extending bore or opening 46 which extends between its ends, means disposed intermediate its ends, such as a flange 48, for aiding in the mounting of the bushing 12 in an opening 50 in the cover 20, and a terminal 22 mounted at its first and external to the'casing 16, which is adapted for connection to the electrical distribution system. Terminal 22 is mounted on end 42 via a gasket member 52, which seals the opening 46 to the atmosphere.

Insulating body member 40 is inserted into opening 50 in the"cover 20, sealing the opening 50 to the atmosphere via a gasket member 54 disposed between the mounting flange 48 and the cover 20. The weather end of the bushing assembly 12 may have a plurality of weather sheds 55 formed therein, to increase the creepage distance from the terminal 22 to the cover 20, and the encased end of the bushing assembly 12 extends into the insulating liquid dielectric. Insulating body member 40 may be secured in the desired position'in opening 50, by providing a circumferential groove 56 in the bushing, between the flange 48 and end 44 of the insulating body member 40, with this circumferential groove cooperating with a spring and flange type mounting assembly (not shown); or, threads may be provided on body member 40 which cooperate with a spring grip type locking nut, for holding the bushing assembly 12 in assembled relation with the casing 16.

An electrically conductive coating 57, such as aluminum paint, is disposed about insulating body member 40, adjacent cover 20, to provide a smooth cylindrical ground shield adjacent the opening in the cover, to. reduce the electrical gradient in this region.

Fuse member 30, which is a fuse of the current limiting type, the operation of which is more fully described in the US. Pat. Nos. 2,496,704, 2,502,992 and 3,134,874, all of which are assigned to the same assignee as the present application, includes an insulating fuse tube 60, formed of any suitable material, such as glass melamine, having first and second metallic ferrules or electrodes 62 and 64, respectively, which may be pressed over and secured to the fuse tube by an suitable adhesive, or otherwise fixed to opposite ends of the fuse tube 60. A fusible element or fuse link 66 is disposed through the opening in the fuse tube 60, and connected between the first and second ferrules 62 and 64, with the fuse link 66 being mounted on an insulating support member 68, if desired. The fuse link 66, which is usually formed of a flat strip or ribbon of silver, has a plurality of spaced notches 70 which extend inwardly from opposite sides of the strip, and periodically reduce the width of the strip, to provide a series of arcs during the operation of the fuse 30, such that the sum of the plurality of arc voltages provide the current limiting effect desired. Arc extinction is provided by filling the fuse tube 60 with a pulverulent or granular arc-quenching material 72, such as silica sandfandthe insulating support member 68 may be formed of an arc-quenching material, if desired, such as a glass polyester, including a suitable filler, such as alum inum'trihydrate, for antitracking characteristics. g

The first and second ferrules have means fixed thereto for connecting the ferrules into the desired electrical circuit, such as axially extending threaded stud members 74 and 76, respectively.

Fuse member 32 is a protectivefuse link, including a cylindrical insulating tubular member 80, which may be formed of an arc extinguishing material, such as fiber, for example, having first andsecond electrodes 82 and 84, respectively, mounted at opposite ends thereof. The inside wall adjacent one end of the fuse tube 80'may be threaded to receive the threads on the outside diameter of the first electrode 82, and the first electrode 82 has a threaded bore or blind opening for receiving the threads of the stud member 76 which is fixed to the second electrode 64 of the current-limiting fuse 30. A fusible element 86 is'disposed in the opening of thefuse tube 80 and connected between the first and second electrodes 82 and 84, and the second electrode 84 is connected to the high-voltage winding 24 via electrical lead 88.

As hereinbefore stated, fuse 32 is electrically connected to the current-limiting fuse 30 by threadably engaging the stud member 76 on the current-limiting fuse 30 with the first electrode 82 of the fuse 32, and the serially connected pair of fuses is mounted in the axial bore 46 of the body member 40 by threadably engaging the first stud 74 of the current-limiting fuse 30 with a suitably threaded blind opening in terminal 22. Thus, the current-limiting fuse is mounted wholly within the bore 46, and the protective link 32 is partially disposed within the bore 46. The protective link 32 is below the surface '18 of the insulating liquid dielectric.

The two serially connected fuses 30 and 32 have coordinated time-current characteristics, with the protective link 32 clearing the circuit upon an overload or short circuit below a predetermined current magnitude, and the current-limiting fuse 30 clears the circuit above this current magnitude. in

other words, their time-current characteristic curves cross one another, with the crossing point establishing the current magnitude at which the current-limiting fuse takes over the function of clearing the circuit from the protective link fuse.

The protective link fuse 32 requires the backup protection of the current limiting fuse 30 in electrical systems which are capable of providing fault current which exceeds the interrupting capacity of the protective link fuse. When the protective link fuse 32 operates, the fault current melts the fusible element 86, and an are, inside a bubble of ionized gas forms along the path of the fusible element. The ionized bubble is the result of the decomposition of the liquid dielectric, such as mineral oil. The intense heat of the arc continues to break down the liquid dielectric, as well as the walls of the fuse tube 80. The breakdown of the liquid dielectric and fuse tube liberates noncondensible, flammable gases having a low dielectric strength, such as hydrogen and carbon monoxide, which increase the pressure within the transformer tank. At the upper limit of the interruption rating of a fusible link, the fusible link cannot clear the circuit, causing the pressure buildup to rupture the tank, or the gas bubble envelopes a live part and ground, causing restrike and continuous arcing. Thus, before this upper limit is reached, the current-limiting fuse reaches its let-through" current magnitude, clearing the circuit, which let-through current magnitude may be well below the maximum available fault current.

When the protective link 32 clears the circuit, the second or lower ferrule 64 of the current-limiting fuse 30 is still at line potential, and may be close enough to grounded parts within the casing, or to the tank wall, that the ionized gas bubble produced by the operation of the protective link 32 in clearing the circuit, may engulf the ferrule 64 and a grounded component within the casing, causing restrike. To prevent this from happening, a flexible heat-shrinkable tubing 90, such as a tubing formed of polyvinyl chloride, is telescoped over the two serially connected fuses 30 and 32, adjacent their intersection, and heated and shrunk about ferrule 64 and the adjacent end of fuse 32, to conform to the adjacent outer surfaces of the two fuse members in a tight-fitting manner, and prevent an ionized bubble from contacting a live part on the fuse members. The flexible insulating tubular member extends for a sufficient distance past the edge 102 of ferrule 64 to ensure that it is adequately insulated, and it also extends along the outer surface of the protective link fuse 32 for a sufficient distance to ensure that the ionized gas cannot reach the ferrule 64 or electrode 82 via this route. An insulating. member 92 is disposed circumferentially about ferrule 64 before the flexible insulating tubular member 90 is shrunk in place, which is dimensioned to center the lower end of the currentlimiting fuse 30 in the opening 46, while still providing a substantial clearance between the ferrule 64 and the inner wall of the opening 46.

The construction of bushing assembly 12 hereinbefore described, was found to be suitable when used at 14.4 kv., but

when the construction was extended for operation at 22 kv., it was found to produce corona discharges which created excessive electromagnetic radiation, i.e., a radio interference voltage of 125 microvolts, when tested at 25 kv.

In the manufacture of electrical distribution transformers, it is desirable to be able to utilize as many components as possible with many different ratings, to reduce the number of different parts which must be either manufactured or purchased and stocked. Thus, when the bushing was found to produce excessive radio interference voltages, the obvious solutions were to completely redesigned the bushing to increase radial and axial clearances, or to fill the bushing cavity with mineral oil, both of which would substantially increase the manufacturing cost of the bushing assembly.

It was found that corona discharges and resulting electromagnetic radiation in the radio spectrum could be substantially reduced, well within the maximum allowable limit, by applying a coating 100 of insulating material, such as an insulating paint, having a thickness as little as 2-4 mils, to the outer surface of current-limiting fuse 30, adjacent to and bridging the intersection of the fuse tube 60 and the ferrule 64. For example, a urethane base paint having a thickness of 3 mils reduced the radio frequency voltage to 9 microvolts at 25 kv., from the 125 microvolts measured when the same bushing assembly was tested at 25 kv. without the coating 100.

It is important to note that this coating 100 of insulating material is disposed between the current-limiting fuse 30 and the flexible tubular member 90. The remarkable reduction in the magnitude of the radio interference voltage level at 25 kv. is believed to be due to the fact that without coating 100, airfilled voids exist between the flexible tubular member 90 and the current-limiting fuse 30, adjacent to the inner edge or corner 102 of ferrule 64, as the heat-shrinkable tubing 90 may not exactly follow the contour of the fuse. Electrical stress is distributed across a strata of elements in inverse proportion to their dielectric constants. Thus, any air trapped adjacent to edge 102 would be more highly stressed when the flexible tubing is applied, than it would be without the tubing, as stress would be transferred into the air space due to the higher dielectric constant of the tubing.

It is believed that by selecting a material for coating 100, such as a urethane based paint which has a dielectric constant at least as large as that of the tubing, i.e., in the range of about 4 to 9, that the area immediately adjacent edge 102 of ferrule 64 may be filled with the coating, providing a smoother contour to which the tubing may more easily conform. The coating 100 is selected to have a higher dielectric strength (volts per mil) than air, and it has a higher dielectric constant than air, which transfers the electrical stresses to the air space outside the fiexible tubular member 90, where the potential gradient is below the electrical breakdown level of air.

Thus, the coating 100, while adding little to the manufacturing cost of bushing assembly 12, enables the bushing assembly 12 to be operated at a much higher distribution voltage, without exceeding the maximum allowable radio interference voltage. The coating of insulating material thus precludes increasing the radial and axial dimensions of the bushing, compared with the dimensions of the bushing found to be suitable for 14.4 kv., and it also makes it unnecessary to construct the bushing such that it will contain oil in its opening or bore.

I claim as my invention: 1. An electrical bushing assembly, comprising: an insulating body member having first and second ends, an opening which extends between its ends, and terminal means mounted at said first end, first and second fuse members, each having an insulating tubular member, first and second electrodes mounted at opposite ends of the tubular member, and a fuse link connected between the first and second electrodes, through the opening in the tubular member, said first fuse member being disposed in the opening in said insulating body member with its first electrode in electrical contact with said terminal means, said second fuse member bemg at least partially disposed in the opening in said insulating body member, with its first electrode in electrical contact with the second electrode of said first fuse member,

an insulating coating disposed on said first fuse member which contacts the outer surfaces of its insulating tubular member and second electrodes adjacent to and bridging their intersection,

and a flexible insulating tubular member disposed in a tightfitting manner about the second electrode of the first fuse member, including said insulating coating, and about a portion of the end of said second fuse member adjacent the second electrode of said first fuse member.

2. The electrical bushing assembly of claim 1 wherein the dielectric constant of the insulating coating is at least equal to the dielectric constant of the flexible insulating tubular member.

3. The electrical bushing assembly of claim 1 wherein the first fuse member is a current-limiting fuse, including pulverulent arc-quenching means disposed to substantially fill the space between its fuse link and the inner walls of the insulating tubular portion of the first fuse member.

4. The electrical bushing assembly of claim 1 including a metallic casing for electrical apparatus having an opening therein, an insulating liquid dielectric disposed in said metallic casing, and means sealingly mounting the insulating body member through the opening in said casing, with the first end of said insulating body member being outside said casing, and its second end, including the fuse link of the second fuse member, extending into said liquid dielectric. 

1. An electrical bushing assembly, comprising: an insulating body member having first and second ends, an opening which extends between its ends, and terminal means mounted at said first end, first and second fuse members, each having an insulating tubular member, first and second electrodes mounted at opposite ends of the tubular member, and a fuse link connected between the first and second electrodes, through the opening in the tubular member, said first fuse member being disposed in the opening in said insulating body member with its first electrode in electrical contact with said terminal means, said second fuse member being at least partially disposed in the opening in said insulating body member, with its first electrode in electrical contact with the second electrode of said first fuse member, an insulating coating disposed on said first fuse member which contacts the outer surfaces of its insulating tubular member and second electrodes adjacent to and bridging their intersection, and a flexible insulating tubular member disposed in a tightfitting manner about the second electrode of the first fuse member, including said insulating coating, and about a portion of the end of said second fuse member adjacent the second electrode of said first fuse member.
 2. The electrical bushing assembly of claim 1 wherein the dielectric constant of the insulating coating is at least equal to the dielectric constant of the flexible insulating tubular member.
 3. The electrical bushing assembly of claim 1 wherein the first fuse member is a current-limiting fuse, including pulverulent arc-quenching means disposed to substantially fill the space between its fuse link and the inner walls of the insulating tubular portion of the first fuse member.
 4. The electrical bushing assembly of claim 1 including a metallic casing for electrical apparatus having an opening therein, an insulating liquid dielectric disposed in said metallic casing, and means sealingly mounting the insulating body member through the opening in said casing, with the first end of said insulating Body member being outside said casing, and its second end, including the fuse link of the second fuse member, extending into said liquid dielectric. 